The present invention concerns a transmitter-receiver device which may be used to transfer information optically. Such a transmitter-receiver device may form part of a communication system for bi-directional transfer of optical signals. The invention also concerns such a communication system.
FIG. 1 shows schematically an example of a bi-directional communication system according to the prior art for transferring optical signals. The system comprises a first transmitter-receiver device A with a receiver unit RXA and a transmitter unit TXA. The transmitter-receiver device A communicates with a similar transmitter receiver device B. Also the transmitter-receiver device B thus comprises a transmitter unit TXB and a receiver unit RXB. The transmitter unit TXB transmits optical signals over a first optical fibre F1 to the receiver unit RXA. In a similar manner, the transmitter unit TXA transmits optical signals over a second optical fibre F2 to the receiver unit RXB. In such a system, the light that is transmitted from the respective transmitter unit TXA, TXB often has an essentially constant average power, i.e. light is normally transmitted all the time. The information transfer is carried out through a suitable modulation of this light signal.
The receiver units RXA, RXB may have an output, UA, UB, respectively, which for example may assume two logical values depending on if the optical power received in the receiver unit RXA, RXB exceeds a certain value. This output UA, UB may for example be connected to an indicator IA, IB, for example in the form of a light emitting diode. Such an indicator IA, IB may for example emit light if the respective receiver unit RXA, RXB receives light. In such a manner, the respective indicator IA, IB may indicate that the connection over the fibre F1, F2 works. The signal from the output UA or UB may also be connected to a network management system NMS. Such a network management system NMS supervises the communication system and makes it possible to, from a completely different position than where the transmitter-receiver devices A and B are located, supervise whether the bi-directional communication system works. The lines 131, 133, 135, and 137 are intended to transfer information carrying signals, for example as electric signals, to and from the transmitter and receiver units RXA, TXA, TXB, and RXB.
In the system according to FIG. 1, for example the transmitter-receiver device B may be arranged in a home and the second transmitter-receiver device A may constitute a centrally located device which transmits signals to the home and receives signals from the home. In order to supervise the system it is desirable to connect the device B to the network management system NMS. If, for example, the device B is positioned in a home or in an office, this device often has no other connection out from the home or the office than via the optical fibres F1 and F2. It is actually conceivable to connect the device B to a network management system via the fibre F1. However, it is relatively expensive and complicated to, in addition to the normal information signals, also transmit signals concerning the network management on the same fibre F1. Furthermore, this network management does not work in case of a breakage of the fibre F1. Since the network management system NMS is intended to supervise the function of the network, it is desirable that this supervision works also in case an error occurs in the communication between the devices A and B. It is, of course, conceivable that the network management system NMS may supervise the device B via another line than via F1 or F2, for example in the home there may be a telephone modem, over which this supervision takes place. This constitutes, however, a complicated solution for transferring signals to the network management system NMS. It is, of course, also possible that a person goes to the device B in order to personally check if, for example, the indicator IB is lit. This may possibly be acceptable within, for example, an office where the distance to B may be short. However, it becomes much more complicated to send a person to the device B if this device is located at a long distance from the position where the person normally is.
As a background to the present invention, also so-called eye-safe fibre communication systems should be mentioned. A problem with fibre communication systems is that the light intensity which is transmitted over the fibres may be relatively high. If, for example, a fibre is broken and if somebody looks into the fibre, damages of the eye might occur. If a fibre is broken or damaged, it is therefore desirable to cut off the light signal which is transmitted over this fibre. For example U.S. Pat. No. 5,136,410 describes such a system.
With reference to FIG. 1 it will now briefly be described how an eye-safe system may work. Suppose that a breakage takes place of the fibre F1. The output UA thereby indicates that no light is received in the receiver unit RXA. A supervising unit (not shown in FIG. 1) may then cut off the transmission of light from the transmitter unit TXA. At the receiver unit RXB it is thereby detected that no light is received. Another supervising unit in connection to the device B thereby immediately cuts off the transmission of light from the transmitter unit TXB. Thereby, no harmful light exists on the broken fibre F1. In order to check if the connection via the fibre F1 works again, the respective transmitter unit TXA, TXB often transmits short light pulses at regular intervals. These light pulses are so short that they are not harmful to the eye. When one receiver unit RXB receives such a light pulse, a similar light pulse is immediately transmitted by the transmitter unit TXB. As long as the breakage of the fibre F1 is the case, the receiver unit RXA does not detect any such light pulse. If, however, the fibre F1 works again, the receiver unit RXA will detect such a response pulse from TXB immediately after TXA having transmitted a pulse to RXB. The connection thus works again and the respective transmitter unit TXA, TXB may now continuously transmit light. The time between the light pulses in such a safety system is usually quite long, for example U.S. Pat. No. 5,136,410 mentions that the time between these pulses is about 49 seconds.
The line 133 (and 135) shown in FIG. 1 may constitute a pair of conductors on which a balanced electric signal is present.
The transmitter unit TXA (and TXB) therefore normally has a transmitter circuit comprising a light source and arranged to operate said light source to transmit optical communication signals in response to electric input signals from a first and second circuit point between which circuit points a balanced electric input signal is intended to be present.
Different transmitter circuits of the above mentioned kind are known. A pair of electric conductors has a certain characteristic impedance for example 100 ohm. In order to avoid undesired reflections, such a pair of electric conductors should in its end point be connected to a load which corresponds to the characteristic impedance.
It should be noted that by a balanced signal is meant that the signal that is present on the pair of conductors is such that the voltages on corresponding points on the two conductors are of the same magnitude but have opposite polarity to a reference potential. This reference potential is usually earth potential. With an unbalanced signal (or “single-ended”) is meant that the signal, i.e. the voltage variation, is only present on one conductor, while the other conductor, or reference potential, is at a constant potential, usually on earth potential.
On a pair of conductors with a balanced signal, due to noise or other phenomena, a signal which is superposed on the two conductors may occur, a so-called common mode signal, which signal may vary with time. This signal is often undesired and should therefore be suppressed. This is often done with the help of, for example, transformers, baluns (a balun is a device which converts a balanced signal to an unbalanced signal) and differential amplifiers.
Also when a balanced electric signal is to be converted to an optical signal, such an undesired superposed signal need to be suppressed in order for the light source, which transmits the optical signal, to be correctly operated. According to the prior art, this has usually been done by first converting the balanced electric signal to an unbalanced electric signal.
FIG. 2 shows an example of the prior art. The electric balanced input signal is here present on a twisted pair 30. The balanced signal is converted to an unbalanced signal with the help of a balun 41 and a transformer 42. The circuit also comprises a termination resistance 43 which is adapted to the characteristic impedance of the twisted pair 30. Thereafter follows one or more circuits 44, which i.a. produce a suitable bias current and a modulation current, wherein the total current drives the light source 20.
Also EP-A-0 542 480 shows an example of a transmitter circuit. The transmitter circuit comprises two differentiators and an amplifier for driving a light emitting diode.
The prior known solutions are relative complicated and expensive, since they often comprise relatively complicated and expensive components, such as active components or transformers. Furthermore, known transmitter circuits often have a relatively high current consumption.
It should be noted that by active components is meant components which produce a gain or a switching, for example transistors, integrated circuits, and diodes.